I seriously doudt after the avengers they all of a sudden they decided to do DOFP.

More likely Singer and Donnor started talking about doing DOFP In 2009/2010 and during
production or post production of first class Singer had Idea to combine first class sequel with DOFP.

The version of DOFP we get in 2014 by SInger Is likely going to be very different than what Vaughn would have done If he hadn't bailed for Secret Service.Just like had Singer been able to direct First Class It would have been different than what Vaughn did.